1. Field of the Invention
The present invention relates to a rotor magnetic pole position detection device in a DC brushless motor drive control apparatus for performing a sensorless drive control under a pulse width modulation (PWM).
2. Description of the Related Art
A conventional DC brushless motor is generallyarranged so that an output circuit including switching elements and so forth chops a DC power to sequentially pass a current through stationary windings of a plurality of phases, whereby a rotary magnetic field is generated to rotate a rotor. A control signal applied to the output circuit at this time is required to be an appropriate signal corresponding to a rotary state of the rotor. To this end, a positional sensor such as a Hall-effect element is provided to detect the rotational position of the rotor. However, recent study and development has been directed to a brushless motor which has a structure of detecting the rotary position of the rotor without using such a positional sensor as a Hall-effect element.
Explanation will be made below as to a principle of detecting the rotary position of the rotor with use of FIG. 12.
FIG. 12 shows voltage waveforms corresponding to one phase under no PWM control in the prior art. In FIG. 12, reference numeral 43 denotes a reference voltage, 44 denotes a waveform of a terminal voltage, 45 denotes an output of a comparator for comparing the reference voltage 43 and the terminal voltage, and 54 denotes a zero-cross point. The principle of detecting the rotary position of the rotor is as follows. The terminal voltage generated in the stationary winding by rotation of the rotor is detected. A phase of the induced voltage in the stationary winding at the zero-cross point (i.e., a positional signal indicative of a predetermined rotary position of the rotor) is detected based on the timing at which the detected terminal voltage coincides with the preset reference voltage.
In the above case, when a continuous ON signal is output during its energization period of the stationary winding as a control signal, the terminal voltage of the stationary winding can be continuously detected. In the PWM control system, however, it becomes impossible to detect the terminal voltage of the stationary winding as it is. In other words, when the PWM control signal which repeats ON/OFF at a fast period during the energization period of the stationary winding is applied, the terminal voltage of the stationary winding can also be detected intermittently as to cross the reference voltage many times as shown in FIG. 13A. Therefore, it is impossible to detect the zero-cross point 54 of the induced voltage, just as it is. FIG. 13A shows waveforms of voltages corresponding to one phase under the PWM control in which a conventional lower arm is switched and waveforms under the PWM control. FIG. 13B shows waveforms of voltages corresponding one phase under the PWM control in which a conventional upper arm is switched. FIG. 15 is a major part of a conventional DC brushless motor drive control device.
The above will be more detailed. As shown in FIG. 15, a DC brushless motor includes a stator having stator coils SU, SV and SW of three phases U, V and W, and a rotor R of a permanent magnet. The DC brushless motor includes a unit (which is called as "upper arm" hereinafter) which has three PNP-type transistors Tr1, Tr2 and Tr3 as switching elements and three flywheel diodes D1, D2 and D3 connected in parallel to these transistors, and a unit (which is called as "lower arm" hereinafter) which has three NPN-type transistors Tr4, Tr5 and Tr6 and three flywheel diodes D4, D5 and D6 connected in parallel to these transistors, as driving circuit for performing the PWM control. The ends of the stator coils SU, SV and SW of the phases U, V and W are connected. The other ends of the stator coils SU, SV and SW are connected to the common connection points between the pairs of the PNP-type transistors Tr1, Tr2 and Tr3 and the NPN-type transistors Tr4, Tr5 and Tr6, respectively. The collectors and emitters of the arms are connected to two bases, and a common connection point ON to the both arms is arranged to output 1/2 of a voltage between the buses.
When such a DC brushless motor is controlled in the PWM manner, this can be realized by performing the PWM control by switching either one of the upper and lower arms using logical operating means such as a micro-computer. The terminal voltage waveform 46 shown in FIG. 13A shows a waveform which is obtained when the lower arm is subjected to the PWM control. When the upper arm is subjected to the PWM control, on the contrary, the terminal voltage waveform shown in FIG. 13B is obtained. To put it simply, these two terminal voltages are inverted mutually with respect to the reference voltage, so that they are basically the same. Accordingly, in view of the fact that the zero-cross point 54 of the induced voltage can not be detected by intermittently detecting the reference voltage crossed many times under the PWM control, the PWM control in which the upper arm is switched is the same as the PWM control in which the lower arm is switched.
Though the above explanation has been briefly made, more detailed explanation will be directed to the waveforms generated when the upper and lower arms are switched. Explanation will first be made as to when the upper arm is subjected to the PWM control. When one (e.g., Tr1) of the three PNP-type transistors as the switching elements in the upper arm is switched (ON/OFF) under the PWM control, a current flows through the stator coils SU and SV during an ON state of the NPN-type transistor Tr5 in the lower arm. At this time, the voltage waveform becomes one shown in duration 1 of the terminal voltage waveform in FIG. 13B. That is, when the switching element is turned ON under the PWM control, a voltage lower by a voltage drop of the element Tr1 than a (+) power voltage of a power source E appears as a U-phase terminal voltage. When the switching element is turned OFF under the PWM control, the electric energy accumulated in the stator coils SU and SV is discharged through the flywheel diode D4. Next, while either one of the remaining two switching elements in the upper arm is in its ON state, the voltage waveform becomes either one of the voltage waveforms in two duration 2 in FIG. 13B. That is, when either one of the PNP-type transistors Tr2 and Tr3 as the switching elements is switched (turned ON or OFF) and is in its ON state and when either one of the NPN-type transistors Tr5 and Tr6 not connected to the elements Tr2 and Tr3 is in its ON state, the induced voltage appears in the terminal of the stator coil SU. When the above switching element is switched to its OFF state, the U-phase terminal voltage shows a neutral point of the terminal voltage between the V- and W-phase terminal voltages (i.e., a value nearly equal to a (-) power voltage of the power source E), so that the voltage in duration 2 has such a shape that its pulse height increases or decreases with time. Further, when the NPN-type transistor Tr4 in the lower arm is turned ON and the PNP-type transistor Tr2 is turned ON or OFF, a voltage higher by a voltage drop of the element than the (-) power voltage appears and has such a waveform shown in duration 3 in FIG. 13B.
Explanation will then be made as to a case where the lower arm is subjected to the PWM control. When one (e.g., Tr4) of the three NPN-type transistors as switching elements in the lower arm is switched (turned ON or OFF) under the PWM control, a current flows through the stator coils SU and SV during the ON state of the PNP-type transistor Tr2 in the upper arm. At this time, a voltage waveform becomes one shown in duration 1 of the terminal voltage waveform in FIG. 13A. That is, when this switching element is turned ON under the PWM control, a voltage higher by a voltage drop of the element Tr4 than the (-) power voltage of the power source E appears as the U-phase terminal voltage. When this element is turned OFF under the PWM control, an electric energy accumulated in the stator coils SU and SV is discharged through the flywheel diode D1. Next, when either one of the remaining two switching elements in the lower arm is in its ON state, a voltage has such a waveform shown in either one of two duration 2 in FIG. 13A. That is, when either one of the NPN-type transistors Tr5 and Tr6 as switching elements is switched (turned ON or OFF) and is in its ON state and when either one of the PNP-type transistors Tr2 and Tr3 not connected to the elements Tr5 and Tr6 is in its ON state, an induced voltage is generated in the terminal of the stator coil SU. When the above element is put to its OFF state under the PWM control, the U-phase terminal voltage becomes a neutral point between the terminal voltages of the V- and W-phases (i.e., a voltage nearly equal to the (+) power voltage of the power source E), so that the voltage in duration 2 has such a waveform that its groove depth increases or decreases with time. Further, when the PNP-type transistor Tr2 in the upper arm is turned ON and when the NPN-type transistor Tr4 is turned ON or OFF, the voltage has a lower value by a voltage drop of this element than the (-) power voltage and has a waveform shown in duration 3 in FIG. 13B.
In this way, although there are the slightly differences in the detailed respects, the PWM control in which the upper arm is switched and the PWM control in which the lower arm is switched are the same.
Explanation will then be made as to how to detect the zero-cross point 54. In order to detect the zero-cross point in the induced voltage of the stationary winding even when a control signal is intermittently output based on the PWM control system and so forth, the following method is considered. That is, during the period in which a PWM control signal is in the OFF state (i.e., during a period in which the induced voltage of the stationary winding can not be detected), it is arranged not to detect a signal calculated by comparing the induced voltage of the stationary winding and the reference voltage.
The above method will be explained using the waveforms in FIG. 13A and the waveforms under the PWM control. In FIG. 13A, reference numeral 43 denotes a reference voltage, 46 denotes a terminal voltage waveform, 47 denotes an output of a comparator under the PWM control, 48 denotes an enable signal, 49 denotes a waveform detected when the comparator output 47 under the PWM control is masked with the enable signal 48, and 54 denotes a zero-cross point. In FIG. 7A, the enable signal 48 which permits the detection only during the period in which the induced voltage of the stationary winding can be detected based on the PWM control signal (i.e., when the control signal is at the H level) is generated. The detection operation is carried out based on the enable signal 48.
The comparator output 47 remains the H level after the zero-cross point because the output is higher than the reference voltage 43. Thus, when the enable signal 48 is applied to the logical operating means, the H level continues to be output. Before the zero-cross point, on the other hand, the enable signal 48 is at the H level during the OFF state of the PWM control signal, so that the comparator output 47 is masked to be at the L level. However, as the H level duration of a counter electromotive voltage caused by the commutation is longer than that of the PWM control signal, the counter electromotive voltage output is at the H level while the PWM control signal is masked to be at the L level.
Although the above explanation has been made as to the case where the lower arm is switched, this also holds true for the case where the upper arm is switched. The comparator output 47 is designed to be turned ON or OFF under the PWM control after the zero-cross point and to become the H level when the enable signal 48 is applied to the logical operating means. As a result, the comparator output 47 is masked to be at the H level. Before the zero-cross point, the comparator output 47 remains the L level, so that it remains its L level even when the enable signal 48 is changed to the H level. In short, a fundamental output waveform when the upper arm is switched is exactly the same as that when the lower arm is switched.
In this way, the DC brushless motor is PWM-controlled in the sensorless mode by detecting the magnetic pole position. The rotor magnetic pole position detection device in the conventional DC brushless motor drive control apparatus under the PWM control will be further explained in more detail. The rotor magnetic pole position detection device in the conventional DC brushless motor drive control apparatus is disclosed in JP-A-5-91790 wherein the PWM control in which an upper arm is switched is performed.
FIG. 14 shows a conventional zero-cross point detection duration of an induced voltage when the PWM control is performed. In FIG. 14, reference numeral 50 denotes a delay time after the PWM control signal is turned ON until the induced voltage starts to be generated, 51 denotes a delay time after the PWM control signal is turned OFF until the induced voltage vanishes, 52 denotes a shift amount from the turning ON of the PWM control signal to the turning ON of the enable signal, 53 denotes a shift amount from the turning OFF of the PWM control signal to the turning OFF of the enable signal, and 56 denotes an attenuating vibration phenomenon. The prior art fails to disclose even a transient response and only describes only such a H level terminal voltage shown in FIG. 14. Although the vibration phenomenon to be inevitably generated is not illustrated in FIG. 14, the purpose in providing the delay times is to avoid the vibration phenomenon, so that a dashed line indicative of the attenuating vibration phenomenon 56 in FIG. 14 is added. For the purpose of setting a duration in which the induced voltage of the stationary winding can be reliably detected, the enable signal is set to be slightly narrower in the duration than an output duration of the induced voltage of the stationary winding, as shown in the same drawing. Thus, the enable signal is generated as follows. The enable signal has a start timing at which the delay time 52 slightly longer than the delay time 50 between the turning ON of the PWM control signal the start of the induced voltage generation elapses. Further, the enable signal has an end timing at which the delay time 53 slightly shorter than the delay time 51 between the turning OFF of the PWM control signal and the disappearance of the induced voltage elapses. In order to provide the delay time 52, a time constant circuit is provided for determining a time constant in an analog circuit.
When such a PWM control signal is input to the drive circuit (upper arm) and the delay time 50 elapses, the induced voltage becomes relatively stable to be at the H level. However, as shown by the dashed line in FIG. 14, when the PWM control signal is applied to the upper arm, on the other hand, a transient current flows through the flywheel diode to maintain the so-far counter electromotive voltage state during the delay time 50 and the attenuating vibration phenomenon 56 takes place at the head part of the induced voltage. When the terminal voltage in the vicinity of the reference voltage influences the output of the comparator, this transient vibration causes the detection failure of the zero-cross point, resulting in the erroneous commutation timing and leading to the cause of stepping-out of the motor. Further, this also causes the fluctuation of the detection timing, involving the increased vibration of the motor.
The conventional magnetic pole position detection device is required to monitor the output signal of the comparator according to the enable signal and not to detect the output signal during the OFF state of the PWM control signal (i.e., during the period in which the induced voltage of the stationary winding can not be detected). However, when logical operating means such as a micro-computer is controlled and uses the enable signal as an interrupt signal, the device must monitor the output signal of the comparator during all the H level state of the enable signal. As a result, the time of the logical operating means to be used for other purposes is decreased, thus reducing the utilization efficiency of the logical operating means.
In this way, in the above conventional DC brushless motor, as the delay time from the PWM control signal when the enable signal is turned ON is determined depending on a given time constant determined by an analog time constant circuit, a relationship between a load and the enable signal is poor. As a result, it has a problem that the attenuating vibration generated in the induced voltage of the stationary winding cannot be reliably avoided when the load is increased. Further, an analog circuit is required for determining a time constant for the delay time generation.
The conventional DC brushless motor also has the following problem when a micro-computer is used as the logical operating means of the magnetic pole position detection device and the enable signal is used as the interrupt signal. During the period in which the enable signal is in the ON state, a zero-cross point can be obtained as the comparison result between the induced voltage of the stationary winding and the reference voltage. As a result, the logical operating means cannot be effectively used for other operations during the period.